1. Field of the Invention
The invention relates to a cross-correlation arrangement for determining the presence or absence of at least one tone signal x.sub.2 (t) of known frequency in an information signal x.sub.1 (t) whose shape and frequency is unknown.
Such arrangements are frequently used in receivers of communication systems, for example for detecting tone signaling signals in telephone systems; for detecting the various frequencies with which telegraph signals are transmitted in an a-synchronous telegraph system and also in radar systems, selective calling systems and paging systems.
2. Description of the Prior Art.
An arrangement as described above is indicated in reference 4 below. In this known arrangement the generation means produce a signal y(t) which is constituted by the sum of the known signal x.sub.2 (t) to be detected and an auxiliary signal a.sub.2 (t) so that: y(t) = x.sub.2 (t) + a.sub.2 (t). Both the auxiliary signal a.sub.1 (t) and the auxiliary signal a.sub.2 (t) are herein constituted by triangular or sawtooth signals.
In another known arrangement of this type (see ref. 2 and 3 of chapter D), auxiliary signals a.sub.1 (t) and a.sub.2 (t) are used which are each formed by a linear combination of signals which change in accordance with Rademacher functions.
In these known arrangements the signals s(t) and y(t) are converted in a particularly simple manner into a digital signal, namely by considering only the sign of these signals. In this way the multiplier means are reduced to a single modulo-2-adder.
For taking an unambiguous decision the condition must yet be set that the signal y(t) or its digital representation is not periodical. By choosing the auxiliary signals in the way as is done in the known arrangements described above, these conditions can be satisfied.
In these known arrangements, the magnitude of the signal produced at the output of the integration means is compared with the reference level in the detection means once in each period T. These detection means for example, supply an "1" pulse if the magnitude of this signal rises above this reference level and in the other case it supplies a "0" pulse. The occurrence of a "1" pulse at the output of the detection means now signifies that the tone signal x.sub.2 (t) is present in the information signal x.sub.1 (t) whilst a "0" pulse indicates that this is not the case.
In these known correlation arrangements the height of the reference level in the detection means is influenced by a plurality of factors. As in the first place the starting instant of the known tone signal x.sub.2 (t) in the information signal x.sub.1 (t) is not known [for example in the communication systems indicated in chapter (A) (1)] the correlation arrangement should continuously be on stand by. The result thereof is that the output signal of the integration means will differ from zero owing to noise signals which are always present in the telecommunication system. The output signal of the integration means produced in response to these noise signals will be hereinafter indicated by noise level. Consequently, the reference level in the detection means must have such a value that it exceeds at all times the noise level. In the second place it is often required that the presence of the tone signal x.sub.2 (t) in the information signal x.sub.1 (t) must be ascertained within a given time interval .tau..sub.0 (the integration time) after the starting instant of the signal x.sub.2 (t). This requirement determines the bandwidth of the discrimination curve. This requirement is furthermore coupled with an additional requirement, namely that a given spread of 2 .DELTA. f.sub.0 must be permissible with respect to the exact frequency f.sub.0 of the tone signal x.sub.2 (t). This means that if the tone signal x.sub.2 (t) received is shifted in frequency over .+-. .DELTA. f.sub.0 this tone signal should yet be recognized within said time .tau..sub.o, as being the known tone signal x.sub.2 (t). Because of these two requirements a value must be attributed to the reference level in the detection means so that the output signal of the integration means should exceed the reference level within said time interval of length .tau..sub.o when a tone signal x.sub.2 (t) having a frequency of .+-. .DELTA. f.sub.o is applied to the arrangement. If a larger value of 2 .DELTA. f.sub.o is required, together with a relatively long integration time .tau..sub.o, then problems arise in the known arrangements because of the fact that a value must be attributed to the reference level used in the detection means which is in the order of magnitude of the noise level. The result of the detection is then strongly affected by the magnitude of the noise signals.